Commutated motor speed measuring means

ABSTRACT

A device for measuring the speed of a commutated electric motor such as a d-c or universal motor uses a current transformer and band-pass filter for sensing power line perturbations caused by motor commutation. The sensed perturbations trigger a one-shot whose ON time with respect to total time is taken as a measure of motor speed. When a universal motor is powered by an a-c power source, a synchronous gating circuit is used to blank the measuring circuit near the zero crossings of the power line. A full wave level detector senses when instantaneous line voltage drops below a preset threshold level and at that time qualifies the synchronous gating circuit. The gating circuit is not actually opened, however, until a subsequent perturbation is detected, at which time motor speed measurement is suspended. When instantaneous line voltage now rises above the threshold level, the synchronous gating circuit is qualified to close but will not close until a second subsequent power line perturbation is detected.

United States Patent Chilton July 4,1972

[ 54] COMMUTATED MOTOR SPEED MEASURING MEANS OTHER PUBLICATIONS E.Bukstein- Industrial Electronics Measurement and Control- H. W. SamsCo.Inc.- 1961-pp. 63, 73- 75, 82

POWER SOURCE Primary Examiner-Michael J. Lynch Attorney-Plante, Hartz,Smith & Thompson, Bruce L. Lamb and William G. Christoforo [57] ABSTRACTA device for measuring the speed of a commutated electric motor such asa d-c or universal motor'uses a current transformer and band-pass filterfor sensing power line perturbations caused by motor commutation. Thesensed perturbations trigger a one-shot whose ON time with respect tototal time is taken as a measure of motor speed. When a universal motoris powered by an a-c power source, a synchronous gating circuit is usedto blank the measuring circuit near the zero crossings of the powerline. A full wave level detector senses when instantaneous line voltagedrops below a preset threshold level and at that time qualifies thesynchronous gating circuit. The gating circuit is not actually opened,however, until a subsequent perturbation is detected, at which timemotor speed measurement is suspended. When instantaneous line voltagenow rises above the threshold level, the synchronous gating circuit isqualified to close but will not close until a second subsequent powerline perturbation is detected.

10 Claims, 3 Drawing Figures COMMUTATED MOTORSPEED'MEASURING MEANSBACKGROUNDOFTI-IE INVENTION This invention relates generally to electricmotor rotational speed measuring devices and more particularly to suchdevices for measuring the speed of a commutated electric motor.

There is a need to measure the rotational speed of both universal andvd-c electric motors without connecting a transducer to the motor orotherwise placing a sensor next to or in proximity with the output shaftof the motor. Furthermore, in many cases the actual motor output-shaftis, not accesible. Additionally, the output motion of variouselectrically powered devices can be other than rotary. For example, theoutput motion of an electrically powered tool may be rotary, orbital, orreciprocating depending on the type of tool. A special sensor willthusbe required for each type. In addition, a

special attachment for positioning of the sensor with respect to themotor driven device might also be required.

It is known that the commutator of an electric motor modu lates theamplitude of the current drawn by the motor. The frequency of thismodulated signal is a function of motor speed and the number ofcommutator segments. Knowing this fact, it would appear to be merelynecessary to sense the power line current perturbations caused bycommutation of the motor and to count these perturbations during adeterminable time period to arrive at a measure of motor speed. However,when the speed of a commutated motor driven by an ac power line, such asa universal 'motor, is to be determined, a second problem presentsitself. Near the zero crossing of the power line, the perturbationscaused by motor commutation may disappear completely or be obscured bycommutation noise, such as would be caused by brush arcing. To eliminatethe uncertainty of measurement near the zerocrossings it is necessary tosense these near zero-crossings and to suspend measurement at that time.

Of course, where a d-c power source is used to drive a universal or ad-c electric motor there will be no near zero crossings and motor speedmeasurement may proceed uninterrupted.

SUMMARY OF THE INVENTION Accordingly, a circuit has been designed whichwill measure the speed of both universal and d-cv electric motorsaccurately whether driven from a-c or d-c power lines. A currenttransformer or other audio frequency current transducer is placed in oneside of the motor power line. The output of this current transducer isconnected to a band-pass filter which removes the power line frequency(if an a-c power line) and the voltage spikes caused by brush arcing.The output of the band-pass filter is an audio frequency signal having afrequency directly proportional to motor speed. The audio frequency isamplified, limited and differentiated to provide a train of triggerpulses at the pulse repetition frequency of the audio signal for aone-shot. In addition, a small amount of hysteresis is provided toprevent false triggering of the one-shot. The one-shot provides aconstant width output pulse regardless of input trigger repetitionfrequency so that the output duty cycle is directly proportional toinput frequency, and hence motor speed, over the operating range of thedevice.

To measure the aforementioned one-shot output duty cycle, a pair ofgating means is provided in addition to a charge storage device. Duringthe one-shot'triggered output pulse the first gating means is opened toallow charges to flow into the charge storage device. During theone-shot quiescent period the second gating means is opened to permitcharges to flow out of the charge storage device. The voltage storedacross the charge storage device, as seen through a ripple filter, is ameasure of motor speed.

To prevent the circuit from measuring near the zero crossing of thepower line both gating means are opened to prevent movement of chargesat the charge storage device during this period. This is accomplished bya full wave level detector which senses when the instantaneous linevoltage drops below a predetermined threshold. A subsequent triggerpulse then opens both aforementioned gating means. When instantaneousline voltage rises above the threshold, as determined by the full wavelevel detector, a subsequent trigger pulse'retums the gating means tonormal operation. Thus, the start of both measure and hold periods aresynchronized with the one-shot trigger pulses. This insures that thecharge storage device will receive an equal and whole number of I chargeand discharge periods and eliminates any error that would result due tothe relative phase of the commutator signal and the level detectoroutput signal.

It is thus an object of this invention to provide a single device formeasuring the rotational speed of a commutated electrical motor drivenby either an a-c or d-c power source.

It is another object of this invention to provide a measuring device ofthe type described which does not make use of transducers connected toor inclose proximity to the motor output shaft.

It is a further object of this invention to provide a motor speedmeasuring means of the type described which makes use solely ofelectrical principles.

These and other objects and features of the invention are described inthe following description in terms of the embodiments thereof which areshown in the accompanying drawings. It is understood, however, that theembodiment described herein is for the purpose of illustration only andis not to be taken as a definition of the limits of the invention.Reference is to be had to the appended claims for this purpose.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTReferring first to FIG. 1 there is seen a power source l0, suitablyeither an a-c or d-c power source, supplying electrical power to thecommutated electric motor 12 via electric transmission lines 13 and 14.An audio frequency current transducer, suitably current transformer 16,has a primary winding 16a serially connected in power line 14 to sensecurrent fiow therein and hence through electric motor 12. The impedancepresented by winding 16a is characteristically very low to avoiddisturbance of the electric motor power. supply. Transducer l6 sensesperturbations in the current flow to electric motor 12, theseperturbations being caused by motor commutation eflects, brush arcing,power line frequency etc. The transfonner secondary winding 16b, havinga transformer resistive load 18 shunted thereacross, sensestheseperturbations and provides a voltage signal acres resistor 18 inaccordance therewith. A band-pas filter 20 senses this voltage signaland allows frequencies close to those expected from consideration ofprobablemotor speeds and commutator configuration to be pmsed. Highfrequency signals which are normally generated by brush arcing andrelatively low frequencies related to power line frequencies areattenuated. The band-pass filter output appears at terminal A. Thewaveform of the signal at this point can be seen at FIG. 2A and is agenerally sinusoidal waveform having a frequency related to the speed ofelectric motor 12. This signal is applied sequentially to amplifier 23and limiter 24, the output of the limiter appearing at tenninal B, thewaveform at that terminal being illustrated at FIG. 23. It can be seenthat the waveform has now been amplified and limited at levels 21 and 22to produce an essentially square waveform. This square waveform isprocessed by the difterminal C, which train of pulses is also seen atFIG. 2C. In this embodiment, only the positive going pulses areutilized. These pulses are applied to one-shot 26 to trigger thatelement to generate a train of its characteristic output pulses. Thispulse train appears at terminal D and is seen in FIG. 2D. As is wellknown, the output pulse from one-shot 26 is a pulse whose duration t,depends on the characteristics of the circuit and which is constant fromone pulse to the other. It should also be obvious that the pulses t,occur at the pulse repetition frequency of the input waveform atterminal A, which it will be remembered is related to motor speed.

With power source an a-c power source and a universal motor connected atmotor 12, switch 35, having switch elements 35a and 35b, is connected asshown, that is with element 35b connecting the high impedance primarywinding 30a of transformer 30 across power lines 13 and 14 and withelement 35a connecting a source of A+ voltage (not shown) at terminal 31to one input tap of comparator 36. Transformer 30 includes splitsecondary winding 30b having a grounded center tap and arranged in ahalf bridge configuration with diodes 32 and 33 whose cathodes areconnected in common at terminal 34, the other input tap of comparator36. A positive voltage signal will be present at terminal 34 only whenthe voltage level across the power lines 13 and 14 is high enough, assensed by transformer 30, to forward bias the diodes. Additionally,comparator 36 will generate an output at its output terminal 37 onlywhen the voltage at its input temiinal 34 exceeds the voltage at itsother input terminal. The elements of this portion of the invention,which can aptly be termed a level detector, can obviously be chosen toprovide a threshold value which the voltage across line 13 and 14 mustexceed in order to cause an output at terminal 37. It can also be seenthat a period of no signal at terminal 37 will occur near the zerocrossings of the power source. As stated, above this threshold a signalappears at terminal 37 and is applied directly to qualify AND gate 40and through inverter 39 to disqualify AND gate 41. A signal generated byAND gate 40 is applied to input tap 44a of flip-flop 44 resulting in aflip-flop generated signal at output terminal 44b together with theextinguishment of any signal then present at flip-flop output terminal44d. An AND gate 41 generated output signal is applied to.flip-flopinput terminal 44c to cause the flip-flop to generate an output signalat output terminal 44d and to extinguish any signal then appearing atoutput terminal 44b.

Assuming now that flipflop 44 is in a first state wherein a signal isbeing generated at terminal 44b, this signal is applied to inputterminal 47a of AND gate 47 to thus qualify that gate. A subsequenttrigger pulse at terminal C triggers one-shot 26 to generate its outputpulse which is applied to the now qualified gate 47 and through inverter66 to P channel field effect transistor 50 to thus render thattransistors drain source circuit conductive during the period of theone-shot output pulse. It can be seen that the drain-source circuit ofFET 50 is serially connected with the drain-source circuit of N channelFET 52 across a d-c reference voltage supply designated V+. The one-shotoutput pulse is applied to OR gate 49 input terminal 49b. In responsethereto the OR gate generates an output which is applied throughinverter 50 to the gate electrode of F ET 52 to thereby render thattransistors drain-source circuit nonconductive.

The common connection between transistors 50 and 52 is connected toterminal E through resistor 55 with capacitor 57 being connected betweenterminal E and ground. Terminal E is connected to the input terminal ofbuffer amplifier 63 which has a characteristically high input impedanceto prevent loading of the circuit consisting of resistor 55 andcapacitor 57. This circuit accordingly acts as a charge storage deviceto store charges delivered by transistor 50 when that transistor isconductive or deliver the charges to transistor 52 when that transistoris conductive or to hold charges when neither transistor is conductive.Additionally, this filter circuit provides filtering between the commonconnection of transistors 50 and 52 and terminal E to remove any ripplewhich might otherwise appear at terminal E. During the one-shot 26quiescent time the signals at input terminals 47b and 49b are removed tothus extinguish the AND gate 47 output signal to thereby deenergizetransistor 50. At the same time the signal fromOR gate 49 isextinguished to thus render transistor 50 conductive across itsdrain-source circuit. Charges now stored in the charge storage circuitwill tend to drain to ground. The filter circuit results in a speedsignal in the form of a voltage at terminal E which is a measure ofmotor speed. The speed signal is impressed, through buffer amplifier 63,across voltmeter 65. The voltmeter is suitably calibrated, by methodswell known to those skilled in the art, to read directly in motorrotational speed.

As aforementioned, at near-zero crossings of the power source, thesignal at terminal 37 is extinguished thus disqualifying AND gate 40 andqualifying AND gate 41 through inverter 39. A subsequent trigger pulseat terminal C not only triggers one-shot 26 but is also now appliedthrough AND gate 41 to flip-flop input terminal 440 thus extinguishingthe signal at output terminal 44b and generating an output signal atoutput terminal 44d. AND gate 47 is thereby disqualified so thattransistor 50 becomes nonconductive. Additionally, the signal at OR gateinput terminal 49a causes transistor 52 to also be nonconductive. Hence,during this time, that is at the nearzero crossings of the power source,no charges will flow into or out of the charge storage device, so thatthe voltage impressed thereacross is held.

When the power line voltage again rises above the threshold, the signalis reestablished at terminal 37. This, of course, disqualifies AND gate41 and qualifies AND gate 40. A subsequent trigger pulse at terminal Cwill now pass through AND gate 40 to input terminal 44a to once againreturn the speed measuring circuit to its normal measuring mode whereinthe output signals from one-shot 26 control the charge and discharge ofthe charge storage device.

For use with a d-c power source where near-zero crossings need not beconsidered, it is merely necessary to turn switch 35 to its secondposition, that is, with element 35b now disconnecting winding 30a fromthe power line and with element 350 connecting the A+ voltage directlyto terminal 37. This permits the speed measurement to be made in amanner identical to that previously described, except that nomeasurement hold periods are introduced. 4

FIG. 3 shows an alternate type of level detector which doesnot require aswitch, such as switch 35 of FIG. 1, to operate the speed measuringmeans with either a-c or d-c motors. Referring to FIG. 3, the powersource 10 here may be either an a-c or d-c source supplying lines 13 and14, seen also in FIG. 1. Line 13 is the grounded line. A voltage divideris comprised of resistors and 71 serially connected between line 14 andground. A second voltage divider is comprised of resistors 73 and 75serially connected between a source of A+ voltage (not shown) and groundwhile a third voltage divider is comprised of resistors 76 and 78serially connected between a source of A- voltage (not shown) andground. The intermediate point of the first divider is connected toinput port 80b of comparator 80 and input port 81a of comparator 81. Theintermediate point of the second divider is connected to input port 800while the intermediate point of the third divider is connected to inputport 81b. The outputs of comparators 80 and 81 are connected,respecu'vely, to the anodes of diodes 83 and 84. The diode cathodes areconnected together and to terminal 37, seen here and in FIG. 1.

A positive voltage reference is thus set at port 80a while a negativevoltage reference is set at port 81b. Whenever the voltage at line 14,as sensed through the first divider, is above the positive reference,comparator 80 generates an output through diode 83 onto terminal 37. Inlike manner, whenever the voltage at line l4, as sensed through thefirst divider, is below the negative reference, comparator 81 generatesan output through diode 84 onto terminal 37. When the sensed voltage isbetween the positive and negative references, no signal appears atterminal 37. It should now be obvious that with power supply 10 an a-csupply, at near-zero crossings of the power line no signal will appearat terminal 37. When the voltage across lines 13 and 14 is not nearzero, either with an a-c or d-c supply at power supply 10, a signal willappear at terminal 37. Thus, the level detector of FIG. 3 will operatein an identical manner as the level detector and switch 35 of FIG. 1 topermit accurate rotational speed measurement of the motor under test.

Although this invention is shown and described herein in particularity,it is to be understood that various changes in the invention may appearto those skilled in the art without departing from the true spirit ofthe invention. Thus, reference is to be had to the appended claims for adefinition of the scope of invention.

The invention claimed is:

1. Means for measuring the rotational speed of a commutated electricmotor energized from a power line whereon current perturbations having arepetition frequency proportional to rotational speed of said motorappear, comprising:

means sensing said current perturbations for generating a train ofpulses at the repetition frequency of said perturbations;

level detector means for generating a hold signal when the voltageacross said power line drops below a preset threshold and forextinguishing said hold signal when the voltage across said power lineis above said preset threshold; and,

means responsive to said pulse train for generating an output signalcorrelated to said repetition frequency while said hold signal isextinguished and for preserving said output signal while said outputhold signal is generated.

2. Measuring means as recited in claim 1 wherein said last named meanscomprises:

means responsive to said pulse train for generating an electricalvoltage level correlated to said repetition frequency while said holdsignal is extinguished and for maintaining said electrical voltage levelwhile said hold signal is generated; and,

utilization means responsive to said voltage level for providing anindication of said motor speed.

3. Means for measuring the rotational speed of an electrical motorhaving a commutator and energized from a power source via conductivemeans comprising:

means sensing current perturbations in said conductive means forgenerating a first signal having repetition frequencies equal to therepetition frequencies of said perturbations;

filter means for effectively removing from said first signalpredetermined frequency ranges;

means responsive to said filtered first signal for generating a train ofpulses standardized as to pulse width at a repetition frequencyproportional to the frequency of said filtered first signal;

charge storage means;

first means responsive to said pulses when in a first state forsupplying charges to said charge storage means;

second means responsive to said pulses when in a second state forremoving charges from said charge storage means, the resultant voltageacross said charge storage means being a measure of said rotationalspeed;

means for generating a second signal when the voltage across saidconductive means is below a preset threshold; and,

means responsive to said second signal for deenergizing said first andsecond means so that charges are neither supplied to nor removed fromsaid charge storage device while said voltage across said conductivemeans is below said preset threshold. 4. Means as recited in claim 3wherein said train of pulses 5 includes identifiable unique points andthe last named means comprises:

means responsive to a first of said identifiable unique points and saidsecond signal for deenergizing said first and second means; and, meansresponsive to a second of said identifiable unique points and saidsecond signal for subsequently energizing said first and second means.5. Means as recited in claim 3 with additionally utilization meansresponsive to said resultant voltage for providing an in- 1 5 dicationof said rotational speed.

6. Measuring means as recited in claim 3 wherein said means forgenerating said train of standardized pulses comprises:

means responsive to said filtered first signal for generating a train oftrigger pulses at a repetition frequency proportional to the frequencyof said filtered first signals; and,

one-shot means triggered by said trigger pulses; and wherein said meansfor generating said second signal comprises means for generating saidsecond signal when the voltage across said conductive means is below apreset threshold and for generating a third signal when the voltageacross said conductive means exceeds said threshold;

means responsive to said second signal and a subsequent one of saidtrigger pulses for deenergizing said first and second means; and,

means responsive to said third signal and a subsequent second one ofsaid trigger pulses for reenergizing said first and second means. 7.Means for measuring the rotational speed of an electric 3 5 motor havinga commutator and energized from a power source via conductive meanscomprising:

means sensing current flow through said conductive means for generatinga first signal having a first repetition frequency related to therepetition frequency of current perturbations; means for generating atrain of pulses standardized as to pulse width and having a secondrepetition frequency related to said first repetition frequency; acharge storage means; first means responsive to said pulses when in afirst state for supplying charges to said charge storage means;

second means responsive to said pulses when in a second state forremoving charges from said charge storage means, a resultant voltageacross said charge storage means being a measure of said rotationalspeed means for generating a second signal when the voltage across saidconductive means is above a preset threshold and for generating a thirdsignal when the voltage across said conductive means is below saidpreset threshold;

means responsive to said third signal and a subsequent pulse of saidtrain of pulses for interrupting the response of said first and secondmeans to said pulses; and

means responsive to said second signal and a subsequent pulse of saidtrain of pulses for reestablishing response of said first and secondmeans to said pulses.

8. Means as recited in claim 7 with additionally utilization meansresponsive to said resultant voltage for providing an indication of saidrotational speed.

* l i il

1. Means for measuring the rotational speed of a commutated electricmotor energized from a power line whereon current perturbations having arepetition frequency proportional to rotational speed of said motorappear, comprising: means sensing said current perturbations forgenerating a train of pulses at the repetition frequency of saidperturbations; level detector means for generating a hold signal whenthe voltage across said power line drops below a preset threshold andfor extinguishing said hold signal when the voltage across said powerline is above said preset threshold; and, means responsive to said pulsetrain for generAting an output signal correlated to said repetitionfrequency while said hold signal is extinguished and for preserving saidoutput signal while said output hold signal is generated.
 2. Measuringmeans as recited in claim 1 wherein said last named means comprises:means responsive to said pulse train for generating an electricalvoltage level correlated to said repetition frequency while said holdsignal is extinguished and for maintaining said electrical voltage levelwhile said hold signal is generated; and, utilization means responsiveto said voltage level for providing an indication of said motor speed.3. Means for measuring the rotational speed of an electrical motorhaving a commutator and energized from a power source via conductivemeans comprising: means sensing current perturbations in said conductivemeans for generating a first signal having repetition frequencies equalto the repetition frequencies of said perturbations; filter means foreffectively removing from said first signal predetermined frequencyranges; means responsive to said filtered first signal for generating atrain of pulses standardized as to pulse width at a repetition frequencyproportional to the frequency of said filtered first signal; chargestorage means; first means responsive to said pulses when in a firststate for supplying charges to said charge storage means; second meansresponsive to said pulses when in a second state for removing chargesfrom said charge storage means, the resultant voltage across said chargestorage means being a measure of said rotational speed; means forgenerating a second signal when the voltage across said conductive meansis below a preset threshold; and, means responsive to said second signalfor deenergizing said first and second means so that charges are neithersupplied to nor removed from said charge storage device while saidvoltage across said conductive means is below said preset threshold. 4.Means as recited in claim 3 wherein said train of pulses includesidentifiable unique points and the last named means comprises: meansresponsive to a first of said identifiable unique points and said secondsignal for deenergizing said first and second means; and, meansresponsive to a second of said identifiable unique points and saidsecond signal for subsequently energizing said first and second means.5. Means as recited in claim 3 with additionally utilization meansresponsive to said resultant voltage for providing an indication of saidrotational speed.
 6. Measuring means as recited in claim 3 wherein saidmeans for generating said train of standardized pulses comprises: meansresponsive to said filtered first signal for generating a train oftrigger pulses at a repetition frequency proportional to the frequencyof said filtered first signals; and, one-shot means triggered by saidtrigger pulses; and wherein said means for generating said second signalcomprises means for generating said second signal when the voltageacross said conductive means is below a preset threshold and forgenerating a third signal when the voltage across said conductive meansexceeds said threshold; means responsive to said second signal and asubsequent one of said trigger pulses for deenergizing said first andsecond means; and, means responsive to said third signal and asubsequent second one of said trigger pulses for reenergizing said firstand second means.
 7. Means for measuring the rotational speed of anelectric motor having a commutator and energized from a power source viaconductive means comprising: means sensing current flow through saidconductive means for generating a first signal having a first repetitionfrequency related to the repetition frequency of current perturbations;means for generating a train of pulses standardized as to pulse widthand having a second repetition frequency related to said firstrepetition frequeNcy; a charge storage means; first means responsive tosaid pulses when in a first state for supplying charges to said chargestorage means; second means responsive to said pulses when in a secondstate for removing charges from said charge storage means, a resultantvoltage across said charge storage means being a measure of saidrotational speed means for generating a second signal when the voltageacross said conductive means is above a preset threshold and forgenerating a third signal when the voltage across said conductive meansis below said preset threshold; means responsive to said third signaland a subsequent pulse of said train of pulses for interrupting theresponse of said first and second means to said pulses; and meansresponsive to said second signal and a subsequent pulse of said train ofpulses for reestablishing response of said first and second means tosaid pulses.
 8. Means as recited in claim 7 with additionallyutilization means responsive to said resultant voltage for providing anindication of said rotational speed.